1. Field of the Invention
The present invention relates to minimizing defects in the components produced by an extreme ultraviolet lithography (EUVL) system, and more specifically, it relates to a method for repairing amplitude defects in an EUVL mask blank.
2. Description of Related Art
Extreme ultraviolet lithography (EUVL) is a technology that employs projection optics to print integrated circuit patterns on silicon wafers at a wavelength of light of approximately 13 nm. Since absorption is high in all materials at this wavelength, the EUVL optics including the mask must be reflective. The EUVL reflective mask blank consists of a thick glass substrate that is first coated with a reflective multilayer film, and then coated with an absorber layer that is subsequently patterned. See C. W. Gwyn et al., J. Vac. Sci. Technol. B 16, 3142 (1998), S. Burkhart et al., Proc. SPIE, vol. 3676, p. 570, 1999 and T. Ikeda et al., “Reflection Type Mask”, U.S. Pat. No. 5,052,033, granted Sep. 24, 1991.
Any defects in the reflective coating or absorber layer are problematic since they produce printing errors in the integrated circuit pattern on the wafer. The basic strategy is to develop extremely clean processes for fabricating the EUVL masks that minimize, and even eliminate, the defect population. However, trends in the current manufacturing of lithography masks suggest that there will be a cost benefit in developing a viable capability to repair a small number of defects on the EUVL mask. The classification (10) of defects that can occur in a EUVL mask is outlined in FIG. 1. Repair methods must be developed for all types of defects.
Referring to FIG. 1, defects (12) in the patterned absorber layer consist of regions where metal is unintentionally remaining or missing. These cause errors in the local amplitude of the reflected field, and hence are “amplitude defects”. There currently exists a mature technology for repairing defects in the absorber layer of lithography masks that work in transmission. It is reasonable to expect that this technology, based on milling and deposition using a focused ion beam, can be extended to repair defects in the absorber layer of EUVL masks. See T. Liang et al., J. Vac. Sci. Technol. B 18, 3216 (2000).
Referring still to FIG. 1, a problem unique to EUVL masks is the existence of defects (14) in the reflective multilayer coating. The prototypical multilayer coating consists of 60 bilayers of molybdenum and amorphous silicon. The thicknesses of the individual layers are approximately 3 and 4 nm for the molybdenum and silicon, respectively. The reflectivity is a resonant property of the coating whereby the fields reflected by every pair of layers interfere constructively. Thus the reflectivity occurs through the depth of the film, and any deformation or disruption of the layer structure within the reflective coating can become a defect.
The classification of defects in the reflective multilayer coating naturally divides into two categories, as indicated in FIG. 1. The first category is the intrinsic-type defect (16). The intrinsic defect is nucleated by the statistical fluctuations that are characteristic of the vapor deposition process that is used to deposit the multilayer film. In particular, there is shot noise in the atom-by-atom deposition process that leads to the accumulation of random roughness. The variance of the roughness scales fairly linearly with the total thickness of the coating. The lower frequency components of the roughness are efficiently replicated by overlying layers and thereby propagate up towards the top of the coating. When one of these random thickness fluctuations exceeds a critical size that is approximately 0.5 nm in height and 100 nm in width, it becomes an intrinsic defect The resulting deformation of the layer structure produces an unacceptable perturbation in the phase of the reflected field. Hence intrinsic defects are “phase defects”.
The second category of defect in the reflective multilayer coating is the extrinsic-type defect (18) as shown in FIG. 1. The extrinsic defect is a deformation or disruption of the multilayer structure nucleated by an external perturbation. This could be a particle, pit or scratch on the mask substrate, a particle imbedded in the multilayer film during the deposition process, or a particle, pit or scratch imbedded on the top of the coating after deposition. As indicated in FIG. 1, the effect of the defect on the reflected field will depend on where the defect is nucleated. When the nucleation occurs at the substrate (20), or in the bottom part of the multilayer coating (22), then the film growth dynamics will tend to damp out the structural perturbation, so that the top layers are deformed but not disrupted. In this case the defect produces a modulation of the phase of the reflected field, and is a “phase defect”. The other possibility is that the defect is nucleated near or at the top of the multilayer coating (24). This could be a particle introduced during the deposition of the top layers, or a particle, pit or scratch imbedded in the top surface subsequent to the deposition. The particle and the damaged part of the multilayer coating will shadow the underlying layers and thereby attenuate the reflected field. Hence these are “amplitude defects”.
U.S. Pat. No. 5,272,744, titled “Reflection Mask”, granted Dec. 21, 1993 by Itou et al. describes a special reticle for x-ray and extreme ultraviolet lithography in order to facilitate the repair of multilayer defects. This reticle is comprised of two multilayer film stacks separated by an Au layer and is in contrast to the conventional reticle design incorporating patterned absorber layers on a multilayer film or the other design of a patterned multilayer on an absorber, as described in U.S. Pat. No. 5,052,033, titled “Reflection Type Mask” by T. Ikeda et al., granted Sep. 24, 1991. There are some disadvantages to the Itou et al. approach, including (i) their reticle is more difficult and expensive to fabricate than other designs, (ii) the introduction of the Au layer will likely introduce additional roughness in the reflective overlayer, reducing the reflectance and throughput of the lithography system, (iii) their repair process is not a local one, and involves covering the entire reticle blank with resist, etc, which could lead to new particulates/defects, and (iv) it is uncertain whether their method will work in a practical sense since it requires extreme control of the Au deposition and various etching processes so that a phase defect does not result from the multilayer defect repair process.
U.S. patent application Ser. No. 09/669,390, titled “Repair of Localized Defects in Multilayer-Coated Reticle Blanks for Extreme Ultraviolet Lithography”, by the present inventors, filed Sep. 26, 2000 and incorporated herein by reference, discloses techniques for repairing multilayer phase defects in EUVL reticles. These techniques utilize a focused energetic beam to induce a contraction in a localized area of the multilayer. When the multilayer structure is significantly disturbed, the defective multilayer alters the amplitude as well as the phase of the reflected EUV light, and the defect is then designated as an “amplitude defect”. The above technique would not be effective for repairing amplitude defects in EUVL reticles; the repair of amplitude defects in EUVL reticles is the subject of this invention.
It is important to develop methods for repairing all types of defects that are anticipated to occur in the multilayer reflective coating. The largest source of defects appears to be the extrinsic defects nucleated by substrate imperfections. A smoothing buffer layer can be deposited between the substrate and the reflective coating to mitigate most of these defects. See P. B. Mirkarimi and D. G. Stearns, Appl. Phys. Lett. 77, 2243 (2000) and U.S. patent application Ser. No. 09/454,715, titled “Mitigation of Substrate Defects in Reticles Using Multilayer Buffer Layers”, by P. B. Mirkarimi et al. filed Dec. 6, 1999. Extrinsic defects nucleated near the bottom of the reflective multilayer coating, as well as all intrinsic defects, will be phase defects. Methods for repairing phase defects in multilayer coatings, based on locally heating the coating or modifying the absorber pattern, are currently under development. See. U.S. patent application Ser. No. 09/669,390, titled “Repair of Localized Defects in Multilayer-Coated Reticle Blanks for Extreme Ultraviolet Lithography”, by the present inventors. The last category of defect that must be addressed is the extrinsic defect that is nucleated near or at the top of the reflective coating, and that modulates the amplitude of the reflected field. The invention that we describe below is a method for repairing this type of amplitude defect.